TFT-type liquid crystal display devices are configured to realize a display operation in such a manner that the voltage applied to the liquid crystal layer (which is electrically referred to as “liquid crystal capacitance”) of each pixel is controlled via a TFT (thin film transistor), whereby the amount of light transmitted through the pixel is adjusted. The polarity of the voltage applied to the liquid crystal layer of each pixel is reversed every predetermined period. Such a driving method for liquid crystal display devices is called “alternate driving method”, in which a DC voltage is not applied across the liquid crystal layer for a long time period. This is because, if a DC voltage is applied across the liquid crystal layer for a long time period, uneven distribution of ions that are present in a liquid crystal material (interfacial polarization) and deterioration of the liquid crystal material occur, so that the display quality deteriorates.
In this specification, the voltage applied to the liquid crystal layer (liquid crystal capacitance) of each pixel is referred to as “pixel voltage”. The pixel voltage refers to a voltage applied between the pixel electrode and the counter electrode of a pixel, which is represented by the potential of the pixel electrode relative to the potential of the counter electrode. When the potential of the pixel electrode is higher than that of the counter electrode, the polarity of the pixel voltage is assumed as positive. When the potential of the pixel electrode is lower than that of the counter electrode, the polarity of the pixel voltage is assumed as negative.
In TFT-type liquid crystal display devices, a pixel electrode connected with the drain electrode of a TFT is supplied with a display signal voltage which is supplied from a source bus line (signal line) connected with the source of the TFT. The difference between the display signal voltage supplied to the pixel electrode and the counter voltage supplied to the counter electrode corresponds to the pixel voltage.
In TFT-type liquid crystal display devices, the polarity of the pixel voltage typically reverses every frame period. Here, in the TFT-type liquid crystal display devices, the frame period refers to a period which is necessary for supplying pixel voltages to all pixels, which means a period extending from a selection of a certain gate bus line (scan line) to the next selection of that gate bus line. The frame period is also referred to as “vertical scan period”. Pixels are arranged in a matrix of rows and columns. Typically, gate bus lines correspond to rows of the pixels, and source bus lines correspond to columns of the pixels. By a scan signal (gate signal) supplied to the gate bus line, TFTs of the pixels are turned on in a row-by-row manner, whereby pixel voltages are sequentially supplied in a row-by-row manner.
In conventional common TFT-type liquid crystal display devices, the frame period is 1/60 second (the frame frequency is 60 Hz). When the input video signal is a NTSC signal, for example, the NTSC signal is a signal for interlaced driving, and one frame (the frame frequency is 30 Hz) consists of two fields, an odd-numbered field and an even-numbered field (the field frequency is 60 Hz). However, in the TFT-type liquid crystal display devices, pixel voltages are supplied to all pixels so as to correspond to respective fields of the NTSC signal, and therefore, the frame period of the TFT-type liquid crystal display devices is 1/60 second (the frame frequency is 60 Hz). Note that, recently, TFT-type liquid crystal display devices of 2×-speed driving (the frame frequency is 120 Hz) and 4×-speed driving (the frame frequency is 240 Hz), for the purpose of improving the movie display characteristics or realizing a 3D display operation, are commercially available. Thus, the TFT-type liquid crystal display devices include a driving circuit which is configured such that the frame period (frame frequency) is determined according to an input video signal, and all pixels are supplied with pixel voltages within each frame period.
In recent years, liquid crystal display devices of the transverse electric field mode, represented by In-Plane Switching (IPS) mode and Fringe Field Switching (FFS) mode, have been used in a wider variety of applications. In liquid crystal display devices of the transverse electric field mode, flickers in the screen which are attributed to the polarity reversal of the pixel voltage are more likely to be perceived than in liquid crystal display devices of the longitudinal electric field mode, such as Vertical Alignment (VA) mode. This is probably because, when the alignment of liquid crystal molecules in the liquid crystal layer undergoes a change which involves a bend deformation or a splay deformation, an alignment polarization occurs due to the asymmetry of the alignment of the liquid crystal molecules. A polarization change which can be caused by such spatial unevenness of the arrangement of dipoles (liquid crystal molecules) is called “Flexo-electric Effect”.
Patent Document 1 discloses a liquid crystal display device which is configured such that a pixel electrode is divided into the first and second regions, the difference between the number of comb teeth in the first region and the number of comb teeth in the second region is one, and the number of comb teeth formed in the pixel region is equal to the number of slits between the comb teeth, whereby the flexoelectric effect is reduced.
Patent Document 2 discloses a liquid crystal display device in which the distribution of an electric field is controlled by, for example, providing a dummy electrode which is parallel to a plurality of band-like portions of the pixel electrode in a region between two adjoining pixel electrodes. Such a solution also enables to reduce the flexoelectric effect.
A low power consumption liquid crystal display device which includes TFTs which include an oxide semiconductor layer (e.g., an In—Ga—Zn—O based semiconductor layer) is commercially available from the applicant of the present application. A TFT which includes an In—Ga—Zn—O based semiconductor layer has high mobility (20 times or more as compared with a-SiTFT) and low current leakage (less than 1/100 as compared with a-SiTFT). When TFTs which include an In—Ga—Zn—O based semiconductor layer are used as the pixel TFTs, the current leakage is small. Therefore, the power consumption can be reduced by applying an intermittent driving method (also known as “low frequency driving method”).
The intermittent driving method is disclosed in, for example, Patent Document 3. According to the intermittent driving method, in the traditional 60 Hz driving (1 frame period= 1/60 second), an image is written in 1 frame period ( 1/60 second) and no image is written in the subsequent 59 frame periods ( 59/60 second), and this cycle is repeated. This intermittent driving method is also known as “1 Hz driving” because an image is written only once within one second. Here, the intermittent driving method refers to a driving method which has a pause period which is longer than a period where an image is written, or to low frequency driving where the frame frequency is lower than 60 Hz.
Whether or not it is likely to be perceived as a flicker depends on a frequency at which a variation of the luminance occurs. For example, even a variation of the luminance which is inconspicuous at 60 Hz is likely to be perceived as a flicker when the frequency is lower than 60 Hz, for example, at 30 Hz or lower. It is known that, particularly when the luminance varies at a frequency near 10 Hz, a flicker is very conspicuous.